The present invention relates to improvements in a transmission for a tractor of so-called three-shaft construction which comprises a main shaft to receive engine power, a travelling speed change shaft disposed in parallel with the main shaft and provided with a travelling speed change means thereon, and a PTO seed change shaft disposed in parallel with said two shafts and provided with a PTO speed change means thereon.
In a prior art transmission as above-described, an engine clutch must always be disconnected when it is intended to change the travelling speed because the travelling speed change means is of a gear type.
Such speed change operation is however very troublesome. Furthermore, when gears or coupling sleeves in the travelling speed change means are slid immediately after the engine clutch has been disconnected, engaging noises may be produced and engagement parts may be broken, or speed change operation cannot be smoothly carried out.
Nowadays when various speed changes and forward-travelling or backward-travelling speed changes are required with the extended applications of a tractor, simplification of such speed change operation is particularly required.
In case a hydraulic clutch type travelling speed change means is employed instead of the gear type one, the above described problem is solved, whereas there occurs another problem that the overall transmission becomes large because a hydraulic type speed change means is generally larger than a gear type one which has the same performance in speed change stages as the former.